Automatic transmissions generally include a transmission control unit (TCU) for automatically controlling shift ratios according to variations in vehicle speed and load. The TCU controls friction elements, a plurality of which are provided in a powertrain, to ON and OFF states such that one of the three elements of a planetary gearset--a sun gear, ring gear, and planet carrier--acts as an input element, another a reaction element, and the last an output element, thereby controlling the number of output revolutions.
To design a powertrain so that four forward speeds and one reverse speed can be output, one compound planetary gearset, comprised of two simple planetary gearsets, and at least five friction elements are used. The compound planetary gearset is structured such that elements from one simple planetary gearset are integrated or shared with elements from the other simple planetary gearset.
However, the powertrain structured as in the above has the disadvantage of having a limited number of forward speeds, i.e., four. As a result, the powertrain cannot make full use of the power provided by engines that are being developed to deliver increasingly higher amounts of output. Also, the restrictive number of forward speeds limits the fuel efficiency of the vehicle.
As a solution, there is provided a prior art powertrain enabling the output of five forward speeds and one reverse speed. Such a powertrain is comprised of three simple planetary gearsets and at least six friction elements.
However, in such a powertrain for a 5-speed automatic transmission, to drive the vehicle in fourth or fifth-speed overdrive, there is a need for an element which rotates at a higher speed than that of a final output element. This results in a substantial amount of power loss. Further, the large number of friction elements results in a heavy and large-size automatic transmission, and acts to increase overall manufacturing costs.